XThunderbird is an extension that provides a full-featured email client, RSS and newsgroup client. It supports different mail accounts (POP, IMAP, Gmail). It has an email account setup wizard, an address book, a user interface with tabs, an integrated spam filter, advanced search, indexing capabilities, and provides an email organization using tags and virtual folders.

The Thunderbird online main features are:

- Message management that can handle several email, newsgroup, and news feed accounts using functionalities such as quick search, message filtering, message grouping, and set labels to find messages.
- Junk filtering that includes a spam filter.
- Protocols supported: POP and IMAP, but it also supports LDAP address completion, RSS/Atom reader, S/MIME and OpenPGP standards for secure email.
- File formats supported: mbox, Unix mailbox format; and maildir, known as maildir-lite.
- Mail Account Setup Wizard, so only providing the name, email address, and password the wizard will check an internal database and find the proper email settings.
- Multiple-channel chat to have real-time conversation with contacts, and supporting multiple networks.
- Activity Manager that records all the interactions between Thunderbird and your email provider in one place.
- NetNews client integrated.
- Multiple folders support.
- Import / export mailboxes.
- Include message queueing.
- Contact management and vCard support.
- Support APOP authentication.
- Support SMTP AUTH.